Vapor generating unit with a plurality of gas passes therefrom



Jan. 18, 1955 P. H. KOCH v 2,699,762

VAPOR GENERATING UNIT WITH A PLURALITY OF GAS PASSES THEREFROM Filed Sept. 28, 1950 3 Sheets-Sheet l f u 4"! I? 34 INVENTOR Fl 6. 1 PM! H Koc/Z BY ATTQRNEY Jan. 18, 1955 P. HfKocH VAPOR GENERATING um" WITH A PLURALITY 0F GAS PASSES THEREFROM 4 3 Sheets-Sheet 2 Filed Sept. 28, 1950 0 2v y 6 n u H m 2 y A 5 V o M H 11 I yflpi m-Mi a F /0 J\ H "I n k 7. W IF INVENTOR Paul H ffivch FIG.2

Jan. 18, 1955 P. H. KOCH VAPOR GENERATING UNIT WITH A PLURALITY F GAS PASSES THEREFROM Filed Sept. 28, 1950 TEMPERA TURE'DEGPEES F 3 Sheets-Sheet 5 GAS TEMP ENTER/Ra SH 1500 AT I400 [300 5/4. 6A5 BY-PASS I 7 7 STEAM TEMPERATURE $.H. 6A6 5)- PASS LOAD PERCENT F I G. 3

INVENTOR Pau/ H K061? ATTORNEY United States PatentO VAPOR GENERATING UNIT WITH A PLURALITY OF GAS PASSES THEREF ROM Paul H. Koch, Bernardsville, N. J., assignor to The Babcock & Wilcox Company, New York, N. Y., a cl'p0, ration of New Jersey Appiication September 28, 1250, Serial No. 187,224 4 Claims. (Cl. 122.-480) This invention relates in general to the. construction and operation of vapor generating and superheating units, and. more particularly, to. units of the type described in Which all or a substantial portion of the vaporgenerating surface. is so located in the combustion chamber of the unit as to receive heat mainly by radiation from the convection superheaters have a vapor outlet temperature curve with a rising characteristic, so that at vapor loads in excess of the. predetermined load, the vapor temperature will be above the desired value. As the associated prime mover is usually designed for operation with a maximum vapor temperature substantially corresponding to the designed vapor temperatureat the predetermined load, provisions are normally made for reducing the vapor temperature to the desired value after the vapor leaves the superheating surface or controlling the vapor superheating conditions in the unit in such a way that the vapor outlet temperature will not exceedthis value.

The rise in vapor superheat temperature at increasing loads is primarily due to the disproportional increase in temperature of the heating gases contacting the superheater with increasing loads. In their passage through the combustion chamber, the gases generated give up a substantial amount of heat. by radiation to the vaporgenerating surface lining the walls of the chamber, The radiant heatabsorbing surface is constant in extent, so that the radiant heat absorbed does netincrease PIOPOI? tionately with the greater volume of heating gases at increased loads. Consequently the temperature of the gases entering the superheater section increases faster than the increase in load. Forthis reason the vapor superheat temperature increases rapidly withincrease in load. This rise'in superheat temperature can be avoided if the increase in heat content of the gases passing to the, superheater is properly limited, and accordingly provisions are made for by-passing a regulable amount of the heating gases around the superheater. Such provisions usually involve a damper-controlled by-pass passage laterally adjacent the superheating surface and arranged to divert a portion of the heating gas stream after the stream-has flowed past and radiatedheat to substantially all of the vapor generating surface in the combustion chamber and over convection heated vapor generating surface in the main gas pass of the unit prior to the vaporsuperheating surface, through the by-pass passage, and thus. out of heating contact with the vapor superheating surface. Such gas by-pass arrangements are elfective where the desired reduction of superheat temperature is relatively low, i. e. where the load range over which superheat temperature is to be maintained at a predetermined value is relatively small.

In accordance with the present invention, vapor generating and superheating units of the general character described are constructed and arranged with an improved location of the fuel burning means and bypass passage relative to the vapor generating surface in the combustion chamber receiving heat by radiation, whereby the unit in which a portion 2,699,762 Eatented Jan. 18, 1955 ice can be operated to maintain a substantially constant superheat temperature over a relatively wide vapor generating load range. More specifically, the fuel burning meansand inlet, to the by-pass passage are so arranged relative to the combustion chamber that when the by-pass passage is in.use,'the heating, gases generated are 'regulably divided in the combustion chamber so that the portions of p the heating gas stream flowing through the main gas pass and by-pass passage respectively will flow in heat exchange relation with different portions of the vapor generating surface in the combustion chamber receiving heat by radiation. As embodied in a commercial unit, the fluid cooled combustion chamber is advantageously vertically elongated and the fuel burning means located at a level intermediate the height of the chamber. With this arrangement, the, inlet to the main gas pass would be located at the upper end of the chamber and the inlet to the by-pass passage in the lower portion of. the chamber substantially below the level of the fuel burning means. A regulable, division of the heating gases generated between the; main gas pass and theby-Pass passage is readily obtainable through control dampers located adjacent the discharge end thereof.

The invention also comprises a modified construction of the vapor superheating surface is located in the by-pass passage. This construction provides an increased control range for a given amount of vapor superheating surface or a given control range with a lesser amount of superheating surface, as compared to a similar construction in which all of the superheating surface is located in the main gas pass.

The invention will be described with reference to a preferred embodiment shown in the accompanying drawings, and other objects of the invention will appear as the description proceeds.

In the drawings: Fig. l is a somewhat diagrammatic sectional elevation a steam generating. unit embodying the invention; Fig. 2 is a view similar to Fig. l of asecond embodiment of the invention; and

Fig. '3 is a chartshowing. curves representing the oper: ation of the embodiments of the invention indicated in Figs. 1 and 2 as compared to the operation of prior art installations.

In Fig. 1- there is shown a large volume combustion chamber 10 having its boundaries defined by steam generating tubes 12, 13, and 14 connected at their upper ends to the steam and water drum 16.and at their lower ends toheaders 18, 20 and 2.1, which may be connected by appropriate downcomers to the water space of the drum 16. Other steam, generating tubes 22 extend from the header 29 along thewall 24 which, in conjunction with the wall delineatedby the tubes 14, defines a superheater gas by-pass 26 having a gas inlet at 23. For this purpose the bypass wall delineated by tubes 14 is. a gas tight walhinchtding suitable closure means between the tubes. Therupper, parts of tubes 22. and the upper parts of some of the tubes 14 form a screen 30 rearwardly of the superheater 31,.and ,continuations of the same tubes extend forwardly toward the drum 16 to delineate a roof 32 for themainrsuperheater gas pass. The lower wall 34 of this gas pass. is defined by intermediate parts of some of the tubes 14. extending tothe screen 3%.

Th y gas inlet of the main superheatergas pass is indicated at 36, this inletbeing disposed at a level well above the level of the burners38 and 40, Whereas the gas inlet for thesuperheater gas by-pass 26 is disposed in the lower part of the combustion chamber and below the level of the burners 38 and 4t].

W1th this arrangement of elements the effectiveness of d fferent portions of the heat absorbing surfaces along the boundaries of the combustion chamber may be regulatedand theheating effects ofthe elements subject to the two different gas flows in the main superheater gas pass and in the by-pass may be controlled. For example,,the heat absorption surfaces in the lower part of the combustion chamber are relatively inactive when there is. no flow offurnace gases through the by-pass 26, and these surfaces become increasingly eifective in their heat absorption when the gas flow from the combustion chamher into the by-pass is increased. For certain rates of firing of the burners for the combustion chamber, the selected variation of gas flow between the inlet 28 of the by-pass 26 and the inlet 36 of the main gas pass not only varies the temperatures of the gases leaving the furnace, but also affects the relative rates of heat absorption beyond those inlets.

The superheater 32 consists of serially connected return bend tubes arranged in banks as shown, and receiving steam from the steam and water drum 16 through circulators 41, an inlet header 42, and inlet connections 44. The superheater outlet header is indicated at 46.

Rearwardly of the superheater and the superheater gas by-pass 26, there is an economizer consisting of a bank of horizontally disposed spaced tubes disposed across the path of gases in the economizer gas pass 52 and the economizer sub-pass 54. The latter is separated from the gas pass 52 by a wall 56 with the exterior walls 58 and 60 forming the opposite boundaries of the by-pass extension 54 and the economizer pass 52. The wall or baffle 56 may be considered as having an extension 57 joining the floor 34 of the main superheater gas pass as clearly shown in Figs. 1 and 2. Below the economizer 50 there are dampers 61-64 with the first of these dampers arranged in the extension 54 of the superheater gas by-pass and the remaining three dampers arranged in the gas pass 52.

The gas flow from the combustion chamber through the extension 52 of the main superheater gas pass and the gas flow from the lower part of the combustion .chamher through the superheater gas by-pass, and its extension are indicated by arrows.

With the arrangement of elements shown in Fig. 1 the superheated steam temperature is controlled by varymg the gas flow in part of the furnace in front of the superheater as well as by controlling the gas flow over the superheater. At a low load the damper 61 will be closed so that no gas will pass through the by-pass 26. Under these conditions, all of the gas passes upwardly through the furnace and over the surface of the superheater 32. At high load the damper 61 may be opened sufficiently to drop the superheated steam temperature to the desired degree. Then, a major portion of the gas will flow up through the upper part of the furnace and over the superheater, and the remainder of the gas will flow down through the lower part of the furnace and then through the by-pass 26.

When the by-pass damper 61 is opened causing a smaller percentage of gas to pass through the upper part of the furnace, the gas temperature entering the superheater will drop, causing the steam temperature to drop. At the same time less gas will pass over the superheater which will also cause the steam temperature drop. For example, by lowering the gas flow over the superheater from 85 to 75%, the steam temperature will drop about 30 because of the gas by-passing the superheater, and an additional 20 drop because of the lower gas temperature entering the superheater, thus affording a total drop of 50 in the steam temperature, compared to a drop of 30 in other gas by-pass installations not having the inlets of the main superheater gas pass and the superheater gas by-pass associated in the manner illustrated. The temperature of the gases entering the superheater pass drops because of the reduced gas flow, the total amount (or area) of pertinent furnace heat absorbing surface being relatively constant.

The modification shown in Fig. 2 of the drawings is similar to that shown in Fig. 1 in most respects. Its main difference is that some of the superheater surface is disposed within the gas by-pass 26. Thus, the superheater designated entirely by the numeral 33 consists of three banks of upright tubes 8082 with the tubes of the banks 80 and 81 extending through the wall 34 and between the sections of the tubes 14 which delineate that wall. The extensions 84 and 85 of the banks of tubes 80 and 81 are disposed within the gas by-pass 26. The object of locating some of the superheater surface in the by-pass 26 is to increase the control range for a given amount of superheater surface or to obtain a given control range with less superheater surface. The Fig. 2 embodiment gives greater superheated steam control range because the gas temperature entering the main superheater gas pass does not drop off as fast with load drop as with other arrangements. This relative increase in gas temperature at low loads is due to a smaller percentage of gas passing through the main superheater pass at full loads, which allows the burners to be placed higher in the furnace.

Comparative gas and steam temperatures are plotted in Fig. 3 against percentage of load, the curves A and A representing a superheater by-pass arrangement of a prior art type (including a main superheater gas pass and a by-pass leading from the same level) and the curves B and B representing operation of the Fig. l embodiment. The curves C and C represent the operating conditions for the Fig. 2 embodiment. In all cases 85% of the gas passes through the main superheater pass at the control point. At full load about 75% of the gas passes across the superheater in the operation of the prior art installation indicated by the curves A and A. This is also true of the curves B and B for the Fig. I arrangement. About 60% of the gas passes across the superheater sections in the main gas pass at full load in the Fig. 2 embodiment which has about 15% of its superheater surface in the gas by-pass.

An inspection of Fig. 3 would show that the Fig. 2 embodiment has a substantially constant steam temperature over a load range extending from 40% of full load to full load, and that this extension of the load range is considerably greater than for the installations which have the characteristics indicated by thecurves A and B.

I claim:

1. In a steam generating and superheating unit, steam generating tubes delineating a vertically elongated combustion chamber, means associated with some of the tubes so as to present therewith a main convection gas pass leading from the combustion chamber at one position near the upper part of the unit and a superheater gas by-pass leading from the combustion chamber at an inlet remote from the gas inlet of the main pass and near the lower part of the unit, some of the steam generating tubes and part of said means defining a heat absorbing wall common to the by-pass and the combustion chamber, a convection superheater including spaced upright tube lengths with the major part of each tube length thereof disposed across the flow of gases in said main gas pass and a minor part of each tube length disposed in the bypass, said means including a wall common to the main pass and the by-pass and having the superheater tubes extending therethrough, fuel burning means firing the combustion chamber at a position intermediate the inlet of the by-pass and the inlet of the main pass, and means for controllably varying the flow of furnace gases through the main pass and the by-pass as the steam generating load changes to maintain a predetermined superheated steam temperature, the inlets being disposed at such widely divergent directions from the fuel burning means that a large proportion of the total heat absorptive surface of the combustion chamber is rendered relatively inactive at low load when there is a minimum of gas flow through the by-pass.

2. In a steam generating and superheating unit, steam generating tubes delineating a vertically elongated combustion chamber, means associated with some of the tubes so as to present therewith a main convection gas pass with its inlet leading from the combustion chamber at its upper part and a superheater gas by-pass leading from the lower part of the combustion chamber, some of the steam generating tubes and part of said means defining a heat absorbing wall common to the bypass and the combustion chamber, a pendent convection superheater having spaced tubes with the major part of their heating surfaces presented by its tubes disposed across the flow of gases in said main gas pass and a minor part of its heating surfaces in the bypass, said means including a wall common to the main pass and the by-pass and having the superheater tubes extending therethrough, individual tubes of the superheater having parts in the main pass and other parts in the by-pass, fuel burning means generating gases for the combustion chamber and disposed at a level intermediate the level of the by-pass inlet and the main gas pass inlet, and means for variably controlling the flow of furnace gases through the main pass and the bypass as the steam generating load changes to maintain a predetermined steam temperature, the inlets being disposed at such widely divergent directions from the fuel burning means that a large proportion of the total heat absorptive surface of the combustion chamber is rendered relatively inactive at low load when there is a minimum of gas flow through the by-pass.

3. In a vapor generating and superheating unit, vapor generating tubes delineating a vertically elongated combustion chamber, means associated with some of the tubes so as to present therewith a main convection gas pass leading from the combustion chamber at one position and a superheater gas by-pass having its inlet leading from the combustion chamber at a position remote from the gas inlet of the main pass, said inlets being disposed near opposite extremities of the combustion chamber, a pendent convection superheater with a major part of the heating surfaces presented by its tubes disposed across the flow of gases in said main gas pass and the remaining part of its heating surfaces in the by-pass, the superheater including a bank of spaced upright tubes with the major part of each of a plurality of the tubes disposed in the main pass and a minor part in the by-pass, said first named means including a wall common to the main pass and the by-pass and having the superheater tubes extending therethrough, fuel burning means generating gases for the combustion chamber and disposed at a position intermediate the inlet of the bypass and the main gas pass inlet, and means for variably controlling the flow of furnace gases through the main pass and the by-pass to maintain a predetermined vapor temperature as the vapor generating load changes, the inlets being disposed at such widely divergent direc tions from the fuel burning means that a large proportion of the total heat absorptive surface of the combustion chamber is rendered relatively inactive at low load when there is a minimum of gas flow through the by-pass.

4. In a vapor generating and superheating unit having vapor generating wall tubes defining a vertically elongated combustion chamber, means including some of said vapor generating wall tubes defining two gas passes having their inlets leading from the combustion chamber at substantially spaced positions one of which is at the upper part of the unit and the other at the lower part of the unit, a bank of spaced tubes constituting a convection superheater disposed in the gas pass leading from the upper part of the unit, the other gas pass constituting a gas by-pass for the superheater, fuel burning means firing the combustion chamber at such a position intermediate the inlets of said gas passes that the gases passing from the fuel burning means to each gas pass inlet are subject to the heat absorption of a substantial and different part of the entire radiant heat absorption surface of the combustion chamber, some of said gas pass forming vapor generating wall tubes defining an upright heat absorbing wall common to the combustion chamber and to said by-pass and having its lower or inlet part disposed within the combustion chamber, means forming parallel economizer passes receiving heating gases from said passes, spaced tubes constituting economizer surfaces disposed in said economizer passes, the superheater by-pass presenting an unobstructed fluid cooled flue leading to its economizer pass, and gas flow control means at the outlet of at least one of the economizer passes, the unit constituting means for maintaining a predetermined final steam temperature of the superheated vapor over a wide load range and over Wide range of furnace firing by controlling gas temperatures at the entrance of the superheater gas pass and simultaneously and by the same act controlling the percentage of gas flow over the superheater.

References Cited in the file of this patent UNITED STATES PATENTS 1,872,138 Grady Aug. 16, 1932 2,007,623 Toensfeldt July 9, 1935 2,147,574 Duram Feb. 14, 1939 2,182,783 Barnes Dec. 12, 1939 2,271,643 Jacobus Feb. 3, 1942 2,590,712 Lacerenza Mar. 25, 1952 FOREIGN PATENTS 525,906 Great Britain Sept. 6, 1940 599,215 Great Britain Mar. 8, 1948 

